1. Field of the Invention
The present invention relates to an inverter circuit adaptable for a discharge lamp load, electronic motor load, high-frequency industrial heating load, antenna load and so forth, and an electrodeless discharge lamp lighting apparatus using this inverter circuit.
2. Description of the Related Art
A conventional inverter circuit is designed to perform switching the output of a fixed frequency oscillator. A conventional electrodeless discharge lamp lighting apparatus uses such an inverter circuit which performs switching the output of a fixed frequency oscillator.
Since the inverter circuit performs switching the output of the fixed frequency oscillator, the frequency associated with the output becomes stable and falls within the desired frequency range. This inverter circuit thus has an excellent feature that no interference will occur in the frequency band that is used in communications.
But this inverter circuit is shifted off the impedance matching status due to a variation in the frequency response of a load containing an inductive or capacitive component (hereinafter referred simply as "load"), which occurs, for example, at the time of lamp starting or with the passage of time. Consequently, the phase relation between the voltage and current output from the inverter circuit changes. This lowers the inverter efficiency and would destroy the device at worst.
An inverter circuit using a variable frequency oscillator may be used instead. However, the frequency of the output of this inverter circuit varies in accordance with a change in the frequency response of the load, so that there is a probability of relatively frequent occurrence of interference in the communications band.
In an electrodeless discharge lamp lighting apparatus, a matching circuit is provided between the inverter circuit and a load constituted of an exciting coil or activation high voltage generating section to make the declination of the load impedance approach zero. This lighting apparatus is however shifted off the matching state due to an adjustment error at the time the apparatus is assembled, a time-dependent change in characteristic, or the like. This causes an excessive amount of reactive current to flow through the device, damaging the device.